The fluids produced by a hydrocarbon well typically comprise a hydrocarbon (oil) phase and an aqueous (water) phase and sometimes a gas phase. One of these phases, often the aqueous phase, is continuous and the other phase is dispersed therein. Knowledge of the proportions of these phases and their flow velocities is required to determine the flow rates from the well of the various phases. Many methods have been proposed for determining flow velocities in single-phase phase or multi-phase flows. One particular approach which is applicable to measuring flows in wells is to introduce tracers into the flow and to measure the passage of these tracers past a measurement station to make a measurement of the flow. One example of a tracer technique is the introduction of a saline solution into the flow and the measurement of the change in electrical conductivity as the tracer passes the measurement station. However, problems can arise due to the natural salinity of the formation water and such a technique only measures the aqueous phase and so cannot be used in isolation to provide all of the required measurements in a hydrocarbon well. As an alternative to saline solutions, radioactive tracers have been used to measure single-phase and multi-phase flows. These tracers can be made either oil-soluble or water-soluble and so the technique can be used to measure both phases in a hydrocarbon well. One example of the use of radioactive tracers to determine water flow behind casing (outside the well) is found in U.S. Pat. No. 3,784,828. An example of a tool used to make such measurements of flow inside hydrocarbon wells is the Tracer Ejection Tool of Schlumberger which is described in U.S. Pat. No. 4,166,215 and U.S. Pat. No. 4,166,216. Minor amounts of suitable radioactive tracer such as iodine 131 are periodically discharged into the continuous-phase well fluid at a selected depth location in the well. Thereafter, by simultaneously measuring the level of radioactivity above and below that location, measurements are obtained which are representative of one or more dynamic flow characteristics of the continuous phase. These measurements are based on the travel time of the tracer from the location where it is discharged into the flow to the measurement stations. Since the ejection of radioactive materials into the fluids that are subsequently produced from the well is often considered undesirable, alternative methods using nuclear radiation techniques have been proposed. These alternative techniques produce short-lived activation components in the flow to provide the radioactive material which is detected, but which is no longer radioactive by the time the fluids are produced from the well. An example of this is found in U.S. Pat. No. 4,233,508 in which the fluid being monitored is irradiated with neutrons such that oxygen atoms are transformed into radioactive nitrogen atoms which decay by emitting .gamma. radiation which is detected at the measurement station. This method of activating a component of the flow only measures the aqueous phase since the oil phase does not include a significant concentration of oxygen atoms which become activated by neutron radiation. Further examples of the use of tracer ejection or activation techniques for measuring flows in wells are disclosed in U.S. Pat. No. 5,047,632 and U.S. Pat. No. 5,306,911.
U.S. Pat. No. 5,543,617 (incorporated herein by reference) discloses a method of measuring the flow velocity of one phase in a multi-phase flow, comprising the steps of creating a nuclear radiation environment around a measurement location in the flowing fluid at which radiation is detected; ejecting a tracer into the flowing fluid upstream of the measurement station which affects detection of the radiation at the measurement location as it passes; making a time-based measurement of the radiation at the measurement location to include passage of the tracer so as to determine the effect of the tracer on the detection of radiation; and using the time-based measurement to determine the flow velocity. A suitable tracer suggested for such a method is a gadolinium-containing compound. Suitable oil miscible tracers include gadolinium brine-in-oil emulsions and Gd tagged organic compounds which can also be oil-soluble. Brine-in-oil emulsions can be prepared using mineral oil, GdCl.sub.3 brines, and a surfactant such as EMUL-HT. A suitable oil-soluble tracer has the general formula Gd(RCOO).sub.3 wherein R is typically CH.sub.3 (CH.sub.2).sub.4. An alternative version of the tracer includes six additional CH.sub.2 groups.
To be effective as a tracer in such a technique, it is necessary that the tracer include a relatively high concentration of gadolinium. It is known that high concentrations of transition metals such as lead, cobalt and manganese can be dissolved in non-polar organic solvents using naphthenic and related acids. The resulting compounds have been used in a variety of applications, including drying agents in paint, insecticidesibiocides and anti-knock additives in gasoline. A common feature of transition metal carboxylates is that their viscosities can be very high, even when dissolved in hydrocarbons at low concentrations; some heavy metal carboxylates have been used as lubricating greases. High viscosity is highly undesirable for a tracer for use in a technique such as that described in U.S. Pat. No. 5,543,617 since it prevents injection of controlled quantities into the flow and dispersion of the tracer throughout the oil phase prior to measurement.
THE ALKALINE-EARTH & HEAVY METAL SOAPS pp 66-69, S. B. Elliot (Reinhold Publishing Corp. NY, USA, 1946) proposes a number of compounds as "dispersion agents" or "flow agents" for use in modifying the viscosity of heavy metal soaps. These compounds have been found to modify the viscosity of the compounds and prolong shelf life. However, none of these applications involve elevated temperatures or conditions similar to those found in oil wells.
Oil-soluble lanthanide compounds have been proposed for various uses. U.S. Pat. No. 4,755,469 discloses the use, as an oil-soluble tracer, of a Group VIB, Group VIIB or lanthanum series rare earth salt of a fatty acid having 5-35 carbon atoms. It is proposed to add the tracer to an oil to be traced and oil samples taken at a remote location and analyzed to see if the tracer, and hence the original oil, is present at that location. The analysis techniques proposed are typical laboratory analyses and the metal in the tracer is chosen so as to be readily distinguished from common formation fluid components. U.S. Pat. No. 4,522,631 discloses a diesel fuel soluble compound of a rare earth metal (including Gd) for use as a fuel performance modifier. The compound typically has 3-25 carbon atoms and metal carbonyls are preferred. These compounds, together with an oxygenated compound such as an alkylcarbitol, aldehyde, ketone, alcohol or ether, are added to the fuel to provide a solution of 0.001-0.1 wt % rare earth in fuel.
None of the prior Gd compounds have been found suitable for use in a tracer flow velocity measurement technique.
It is an object of the present invention to provide a Gd compound in a form which is oil-soluble and has sufficient Gd content to be useful as a tracer in flow velocity measurement technique. Such a tracer ideally will have relatively low viscosity which is retained even though the tracer may be subjected to elevation of temperature or temperature cycling prior to use.